Proteoglycans are glycoproteins that contain one or more sulfated glycosaminoglycan (GAG) chains. They are classified according to their core protein.

Glycosaminoglycans are usually classified into five types: chondroitin sulfate, dermatan sulfate, heparin sulfate, heparin, and keratin sulfate. Glycosaminoglycans  make up part of proteoglycans.

Proteoglycans are synthesized by all connective tissue cells. They can remain associated with these cells on their cell surface, intracellularly or in the basement membrane. These cell-associated proteoglycans commonly contain hepain/heparin sulfate or chondroitin sulfate as their major glycosaminoglycans.

Alternatively, proteoglycans can be secreted into the extracellular matrix . These matrix proteoglycans usually contain chondroitin sulfate, dermatan sulfate, or keratan sulfate as their major GAGAs.

Decorin helps bind type II collagen fibers together in cartilage, while fibromodulin and lumican bind type II collagen to type IX.

Proteoglycans are degraded by proteinases, which release the glycosaminoglycans. The glycosaminoglycans are taken up by cells by endocytosis, where they re degraded in lysosomes by a series of glycosidases and sulfatases.

Defects in these degradative enzymes can lead to diseases called mucopolysaccharidoses.